1. Field Of The Invention
The invention relates to metal forming and stamping machines in which opposed punch and die tools are brought together on a malleable work piece such as a length of sheet metal. According to an aspect of the invention, cooperating tools such as a punch and complementary die are carried on opposed tool carrying structures that are independently complementary, namely being engaged with one another around a periphery for guidance in a male-female guiding arrangement that precisely and repeatably determines the relative positions of the punch and die tools as they are brought together at the work piece in the stamping operation.
2. Prior Art
In a conventional technique for the manufacture of certain articles, particularly from flat strips of metal, blank stock is fed into a specially manufactured tool called a stamping press, which carries a stamping die. The stamping die and other associated structures have shaped forming edges and surfaces that respectively support or strike the stock. Through the pressure and motion of the device, the tool cuts out and/or forms the stock into the metal parts or components of the required size and shape.
The stamping press has driving elements that impart the necessary motion and tonnage to force the flat metal strip into the desired shape in conjunction with the tool structures, typically moving one of two interengaging tools in an oscillatory displacement against an opposed tool which is mounted on a stationary support. Typically the motion involves the vertical oscillation of a forming tool against a fixed tool on a horizontal bed. One or a number of forming operations can be accomplished in a stroke, plural operations being accomplished by placing tools configured for successive forming steps laterally adjacent to one another along a feed path of the stock.
Stamping presses are available in a wide variety of sizes and capabilities, depending upon the size and complexity of the required parts, as are the tools. Stamping presses can produce small parts at very high rates, and may operate at over 3,000 vertical strokes per minute. Metal stamping dies are used to manufacture parts ranging from very small and/or sophisticated components for the electronics industry, to large shapes such as portions of an automotive body.
Metal stamping dies typically comprise two associated halves, which together are referred to as a "punch and die set." A conventional punch and die set has an upper shoe and a lower shoe to which the metal forming tools are mounted, and guide posts and bushings for holding the upper and lower shoes, and therefore the tools, in precise alignment during die construction, die maintenance, die setting and in use to form parts.
The number of guide posts and bushings varies according to the accuracy required and the size of the die set. In a typical vertically-oriented stamping die, the upper half of the stamping die holds a punch-set, and is usually the portion that is attached to the movable part of the stamping press, known as a "ram." The ram moves the punch-set up and down relative to the lower half of the stamping die (referred to as the die-set), which is stationary on a heavy bolster plate defining a fixed bed. Metal stamping dies perform different processes upon the flat metal strip, depending on the configuration of the surfaces that contact the work piece. The processes include, without limitation, coining, drawing, blanking, piercing, notching, embossing, and similar types of bending, forming and cutting operations, etc. The tools that perform the foregoing processes require machining and mounting to very precise tolerances. For example, to cut through the work piece an appropriate clearance between passing edges of the punch and die tools might be less than two tenths of one thousandth of an inch. Depending on the thickness of the stock and other parameters, a relatively wider clearance between the same passing edges may not cut through the stock at all, and instead will bend or draw the stock without parting it. For dependable correct operation, the dimensions and relative positions of the punch and die tools can be critical. In addition, a lack of sufficient clearance, or even contact between punch and die portions that are intended to pass one another, can wear or destroy the punch and die sets.
The tolerances can be very demanding. A punch and die set is preferably capable of being precisely opened and closed and will correctly form parts repeatedly, for hundreds of thousands of successive strokes of the stamping press. The punch and die set structures must be precisely and accurately positioned in the stamping press and the movable and stationary parts of the press that carry the punch and die sets must be moved precisely and accurately to ensure that the punch and die structures meet and/or pass in the exact same relative position each and every time the stamping press cycles. This is necessary to prevent the stamping die from damaging itself as well as to correctly form the parts.
There are potential problems associated with punch and die sets that can cause finished part quality problems and punch and die set tool damage. A very reliable, extremely precise, and accurate alignment design is required in order to ensure that a stamping die is always correctly aligned. If a punch and die set is not aligned correctly, for whatever reason, expensive damage will occur. For example, punch tools and die sections can chip or break, or excessive wear of the tools will occur, necessitating frequent stoppages for maintenance.
On the other hand, the press driving and guiding structures must be heavy and durable if they are to survive a large number of operations. It can be all the more difficult to ensure accurate and repeatable motion in very heavy driving arrangements needed for durability and longevity. With these needs in mind, various methods have been employed in the prior art to prevent or reduce misalignment of punch and die tools.
Known stamping dies and presses typically comprises planar supporting structures such as a movable rectangular plate on the punch side and a stationary horizontal bed or support for the die. Driving mechanisms bring the supporting structures together relative to one another in a direction normal to the parallel planes of the movable plate and the bed. For guidance, the punch and die sets are usually connected, respectively, to the ram and to the bed or base of the stamping press and to one another, including by two, three or four vertical guide posts, for example at the corners of the punch and die set. The posts can be fixed in one of the movable and stationary sides and movable in an opening in the other. These vertical posts also can be configured in a male/female elongated arrangement with the top or punch-posts fitting over or inside the lower die-posts. p Both the stamping press and punch and die set depend upon the guide posts for alignment of the punch tools with the die tools, and the punch and die set with the stamping press ram. A number of guide posts are provided at positions spaced laterally from one another, defining a line along which the movable and stationary members come together, and fixing the relative orientation of the movable and stationary members as well.
A punch and die set may have inherent alignment problems due to their shape. For a punch and die set to make a viable component, the top half (e.g., the punch) must meet the bottom half (die) in a nearly perfectly parallel orientation (usually level horizontal). If, for example, one corner is lower than another, the angular error results in a corresponding side-to-side or front-to-back lateral error in the relative positions of interacting punch and die surfaces and edges. Even the slightest difference in angular relationship between the punch and die may damage the metal forming tools or result in poor quality parts.
A four-post die, punch and die alignment is considered appropriate to maintain relative positions and to prevent uneven closing of the punch and die set. The posts are typically located at or near the periphery of the bed, such as at the corners of a rectangular bed. In this arrangement, the guiding interactions of the individual posts are well spaced from one another, which spreads the guiding effect of the posts and tends to provide more positive support than a closer array or fewer guide posts. However, each post involves a limited mass of material and has a relatively small surface area in contact with the bore in the opposed structure or the like. The posts and their bores can wear at different rates, for example due to die imbalance or uneven buildup of grease and dirt along each post. Furthermore, with wear, the plates can tend to move out of a parallel orientation and become canted. This can cause uneven performance of the stamping die as well as an increased rate of wear on the guiding mechanism, resulting in increasingly poor or variable quality in the stamped parts.
Apart from the aspect of angular error between the punch and die set, the overall alignment of the press can vary and negatively affect the performance of the punch and die set, particularly where highly demanding tolerances are involved. For example, if the press ram has incurred an angular error, the thrust vector for the ram's vertical motion is not exactly perpendicular to the base of the press, this error in the alignment of operative forces is likewise translated to the punch and die set.
Prior art designers have developed methods of loosely suspending a punch and die set from the ram to eliminate a direct angular error. This improvement in alignment separates the matter of press alignment form punch/die plate alignment but has other drawbacks. Substantial complexity must be added to the punch and die arrangement, and even greater reliance for structural support and alignment is placed on the four guide posts. Thus suspending the punch to relieve ram alignment sensitivity may introduce angular errors by increasing wear and reliance on the guide posts. As a practical matter, this concept works only if every other component is in excellent condition and operates perfectly, and the press is only out of alignment by a minor span, such as a few thousandths of an inch. Such circumstances rarely occur in normal manufacturing.
Press manufacturers also have attempted to add dimensional stability and/or to reduce alignment errors by adding margins of safety to the various die and punch components. These efforts add cost and complexity to the punch and die.
Improvements in the prior art to improve the alignment of punch and die sets with time and wear, have added substantially to the complexity of an already complicated technology. The practical improvement in accuracy has been modest by comparison. There is a need in the art to reduce or eliminate angular alignment errors and improve positioning accuracy, without adding substantial complexity to the structure of dies and presses. Conversely, there is a need for a less complicated and expensive arrangement for punch and die sets that is at least as accurate for punch/die positioning, and preferably is more accurate, than the conventional structure in which a plurality of spaced guide posts constrain the relative motion of the punch and die and their carrying structures.